Microphysics of Largescale Clouds

Major improvements have recently been made in the description of cloud microphysics for large-scale models. Although early studies diagnosed cloud amount based on relative humidity, most current global climate models explicitly calculate cloud condensate in large-scale clouds. The degree of sophistication varies from calculating the sum of cloud water and ice to calculating cloud water, cloud ice, snow, and rain as separate species (Lohmann and Feichter 2005). Because the aerosol indirect effect is based on the change in cloud droplet number concentration, some global climate models calculate explicitly cloud droplet number concentrations in addition to the cloud water mass mixing ratio using one of the above described physically based aerosol activation schemes as a source term for cloud droplets. Similarly, the number of ice crystals needs to be calculated in addition to the ice water mass mixing ratio to estimate the effect of aerosols on mixed-phase and ice clouds. Determining the size-dependent sedimentation rate of hydrometeors requires at least a two-moment scheme. Representing size-dependent sedimentation leads to important differences in the cloud vertical structure, cloud lifetime, and cloud optical properties. Two-moment schemes are superior to one-moment schemes provided that the second moment can be treated adequately. Major uncertainties remain, however, in terms of cloud droplet activation and precipitation formation, as discussed below. Theoretically, the best approach would be to use a size-resolved treatment of the cloud microphysics. However, using this approach in a global climate model would be questionable, because treatment of cloud dynamics, including entrainment and advection, is not accurate enough to warrant this level of detail.

0 0

Post a comment